This invention relates generally to techniques for alignment of shafts of rotating equipment. More particularly, the invention is concerned with a technique for aligning the shafts of interconnected machines.
The procedures are well known for accomplishing alignment of the mating shafts of two pieces of interconnected machinery such as a turbine which drives a compressor. See for example an article by C. Jackson in Hydrocarbon Processing for September, 1971, pages 189 to 194 entitled "How to Align Barrel-Type Centrifugal Compressors". See also an article by Jack N. Essinger also in Hydrocarbon Processing for September, 1973, pages 185 to 188 entitled "A Closer Look at Turbo Machinery Alignment".
Misalignment between the coupled-together driving and driven shafts of two or more items of rotating equipment creates many problems such as vibration, failed couplings, reduced thrust and the like. In order to avoid such misalignment, a well-known prior art technique for the alignment of two such interconnected shafts is the so-called "reverse indicator" procedure. In this procedure, an elongated alignment bracket or bar is attached to each shaft extending in the direction of the opposite shaft across the gap constituting the coupling spacer length usually about 60 inches. The free end of each bracket is provided with a dial indicator, the stem of which makes normal contact with the OD of the opposite shaft. In accordance with standard procedure, the two shafts are rotated simultaneously through precise 90.degree. increments. Dial indicator readings are taken at each position on the OD of both shafts. These readings may be recorded on what is known as an alignment map in the cold alignment position and thereafter in a hot equilibrium position at which point the two shafts should be in perfect alignment. If not, some shim corrections may have to be made under one or both pieces of equipment and the readings taken again.
One difficulty in the performance of this procedure is that the shafts must be turned in one direction only. If the precise degree position is overshot, one must continue rotation in the same direction until precisely the proper angular position is again reached. With conventional manual turning devices, it is very difficult to rotate the shaft of a high inertia device. For example, a large turbine may have a breakaway inertia of 2500 foot pounds. The corresponding momentum of such a turbine shaft makes it extremely difficult to avoid overshoot and to stop the shaft at precisely 90.degree. rotational intervals to within plus or minus a quarter of a degree. Not only do these problems make accurate alignment difficult, but the procedure of taking multiple reverse indicator alignment readings for a single pair of shafts can easily take many hours.
A typical manual shaft turning technique is to bolt one end of a straight piece of pipe to the shaft hub and have two men apply torque by means of a long "cheater bar" which is passed through transverse holes at the other end. This is unsatisfactory for several reasons. First, if the pipe is attached to the same bolt circle as the alignment fixture, its removal may disturb the dial indicator readings and force the turning crew to start over. Also, the use of such a cheater bar is dangerous. If the momentum of the shaft, once started, is so great that there is no time to remove the cheater bar, it will be carried around and driven with force against the adjacent floor. The potential hazard to personnel is obvious.
A further problem encountered in the reverse indicator shaft alignment procedure is that of so called "bracket sag". Alignment brackets typically consist of constant diameter pipe which inevitably deflects several mils between its two ends. Careful alignment work requires that this deflection be accurately determined and that appropriate corrections be made in the data.
It is therefore a general object of this invention to provide an improved procedure for the alignment of shafts of interconnected rotating equipment.
It is a more particular object of this invention to provide a procedure for shaft alignment with greater economy of time and safety than heretofore realized together with improved accuracy.
Other and further objects and advantages of this invention will become apparent from a consideration of the detailed description of the drawings to follow, taken in conjunction with the appended claims.